Thermal Transport in LEDs for Solid State Lighting

Over 20% of electricity in US is used by lighting. Solid state lighting (SSL) efficiency can theoretically surpass that of incandescent and fluorescent lighting techniques. Nonetheless SSL efficiency is greatly reduced at high temperatures that result from inadequate heat dissipation. SSL requires blue and green light emitting diodes (LEDs) made from Gallium Nitride (GaN) and Indium Gallium Nitride (InGaN) to eventually generate white light. Using an infrared thermal imaging camera, temperatures of working blue and green LEDs with different efficiencies were measured. The results show that higher efficiency LEDs have lower active region temperatures when driven with the same power. Further, they motivate our study of thermal properties of the individual thin films that compose the LEDs, since earlier studies show that conduction is the primary dissipative mechanism for heat in LEDs. Bulk thermal properties are poor estimates of thin film properties due to increased boundary and defect scattering of phonons in the films. By examining real LED structures with the 3-omega technique, thin film thermal conductivities can be measured. For this technique, a thin metal line was fabricated onto a smooth dielectric sample surface. This thin metal line works as both a heater and a thermometer. Benchmark studies on Pyrex 7740 were used to validate our 3-omega setup. Data from real GaN/InGaN LED structures show that the effective thermal conductivities of the AlN buffer layer and multi-quantum-well active region are substantially suppressed relative to their anticipated values based on bulk properties.